Ceramic composite bodies with increased metal content

ABSTRACT

This invention relates generally to a novel method for forming a self-supporting body. Specifically, the formed self-supporting body has a higher volume percent of metallic constituent relative to a body formed by similar techniques. A first porous self-supporting body is formed by reactively infiltrating a molten parent metal into a bed or mass containing a boron donor material and a carbon donor material (e.g., boron carbide) and/or a boron donor material and a nitrogen material (e.g., boron nitride) and, optionally, one or more inert fillers. Additionally, powdered parent metal may be admixed with a mass to be reactively infiltrated to form additional porosity therein. The porous self-supporting body which is formed by the reactive infiltration process according to this invention should contain at least some interconnected porosity which is capable of being filled in a subsequent step with additional metal, thus increasing the volume percent of parent metal in the body at the expense of porosity.

TECHNICAL FIELD

This invention relates generally to a novel method for forming aself-supporting body. Specifically, the formed self-supporting body hasa higher volume percent of metallic constituent relative to a bodyformed by similar techniques. A first porous self-supporting body isformed by reactively infiltrating a molten parent metal into a bed ormass containing a boron donor material and a carbon donor material(e.g., boron carbide) and/or a boron donor material and a nitrogen donormaterial (e.g., boron nitride) and, optionally, one or more inertfillers. Additionally, powdered parent metal may be admixed with a massto be reactively infiltrated to form additional porosity therein. Thefirst porous self-supporting body which is formed by the reactiveinfiltration process according to this invention should contain at leastsome interconnected porosity which is capable of being filled in asubsequent step with additional metal, thus increasing the volumepercent of parent metal in the body at the expense of porosity.

BACKGROUND ART

In recent years, there has been an increasing interest in the use ofceramics for structural applications historically served by metals. Theimpetus for this interest has been the superiority of ceramics withrespect to certain properties, such as corrosion resistance, hardness,wear resistance, modulus of elasticity, and refractory capabilities whencompared with metals.

However, a major limitation on the use of ceramics for such purposes isthe feasibility and cost of producing the desired ceramic structures.For example, the production of ceramic boride bodies by the methods ofhot pressing, reaction sintering and reaction hot pressing is wellknown. In the case of hot pressing, fine powder particles of the desiredboride are compacted at high temperatures and pressures. Reaction hotpressing involves, for example, compacting at elevated temperatures andpressures boron or a metal boride with a suitable metal-containingpowder. U.S. Pat. No. 3,937,619 to Clougherty describes the preparationof a boride body by hot pressing a mixture of powdered metal with apowdered diboride, and U.S. Pat. No. 4,512,946 to Brun describes hotpressing ceramic powder with boron and a metal hydride to form a boridecomposite.

However, these hot pressing methods require special handling andexpensive special equipment, they are limited as to the size and shapeof the ceramic part produced, and they typically involve low processproductivities and high manufacturing cost.

A second major limitation on the use of ceramics for structuralapplications is their general lack of toughness (i.e. damage toleranceor resistance to fracture). This characteristic tends to result insudden, easily induced, catastrophic failure of ceramics in applicationsinvolving even rather moderate tensile stresses. This lack of toughnesstends to be particularly common in monolithic ceramic boride bodies.

One approach to overcome this problem has been to attempt to useceramics in combination with metals, for example, as cermets or metalmatrix composites. The objective of this approach is to obtain acombination of the best properties of the ceramic (e.g. hardness and/orstiffness) and the metal (e.g. ductility). U.S. Pat. No. 4,585,618 toFresnel, et al., discloses a method of producing a cermet whereby a bulkreaction mixture of particulate reactants, which react to produce asintered self-sustaining ceramic body, is reacted while in contact witha molten metal. The molten metal infiltrates at least a portion of theresulting ceramic body. Exemplary of such a reaction mixture is onecontaining titanium, aluminum and boron oxide (all in particulate form),which is heated while in contact with a pool of molten aluminum. Thereaction mixture reacts to form titanium diboride and alumina as theceramic phase, which is infiltrated by the molten aluminum. Thus, thismethod uses the aluminum in the reaction mixture principally as areducing agent. Further, the external pool of molten aluminum is notbeing used as a source of precursor metal for a boride forming reaction,but rather it is being utilized as a means to fill the pores in theresulting ceramic structure. This creates cermets which are wettable andresistant to molten aluminum. These cermets are particularly useful inaluminum production cells as components which contact the moltenaluminum produced but preferably remain out of contact with the moltencryolite.

European Application 0,113,249 to Reeve, et al. discloses a method formaking a cermet by first forming in situ dispersed particles of aceramic phase in a molten metal phase, and then maintaining this moltencondition for a time sufficient to effect formation of an intergrownceramic network. Formation of the ceramic phase is illustrated byreacting a titanium salt with a boron salt in a molten metal such asaluminum. A ceramic boride is developed in situ and becomes anintergrown network. There is, however, no infiltration, and further theboride is formed as a precipitate in the molten metal. Both examples inthe application expressly state that no grains were formed of TiAl₃,AlB₂, or AlB₁₂, but rather TiB₂ is formed demonstrating the fact thatthe aluminum is not the metal precursor to the boride.

U.S. Pat. No. 3,864,154 to Gazza, et al. discloses a ceramicmetal systemproduced by infiltration. An AlB₁₂ compact was impregnated with moltenaluminum under vacuum to yield a system of these components. Othermaterials prepared included SiB₆ --Al, B--Al; B₄ C--Al/Si; and AlB₁₂--B--Al. There is no suggestion whatsoever of a reaction, and nosuggestion of making composites involving a reaction with theinfiltrating metal nor of any reaction product embedding an inert filleror being part of a composite.

U.S. Pat. No. 4,605,440 to Halverson, et al., discloses that in order toobtain B₄ C--Al composites, a B₄ C--Al compact (formed by cold pressinga homogeneous mixture of B₄ C and Al powders) is subjected to sinteringin either a vacuum or an argon atmosphere. There is no infiltration ofmolten metal from a pool or body of molten precursor metal into apreform. Further, there is no mention of a reaction product embedding aninert filler in order to obtain composites utilizing the favorableproperties of the filler.

While these concepts for producing cermet materials have in some casesproduced promising results, there is a general need for more effectiveand economical methods to prepare boride-containing materials.

DESCRIPTION OF COMMONLY OWNED U.S. PATENTS AND PATENT APPLICATIONS

Many of the above-discussed problems associated with the production ofboride-containing materials have been addressed in U.S. Pat. No.4,885,130 (hereinafter "Patent '130"), which issued on Dec. 5, 1989, inthe names of T. Dennis Claar, Steven M. Mason, Kevin P. Pochopien, DannyR. White, and William B. Johnson, and is entitled "Process for PreparingSelf-Supporting Bodies and Products Produced Thereby".

Briefly summarizing the disclosure of Patent '130, self-supportingceramic bodies are produced by utilizing a parent metal infiltration andreaction process (i.e., reactive infiltration) in the presence of a masscomprising boron carbide. Particularly, a bed or mass comprising boroncarbide and, optionally, one or more of a boron donor material and acarbon donor material, is infiltrated by molten parent metal, and thebed may be comprised entirely of boron carbide or only partially ofboron carbide, thus resulting in a self-supporting body comprising, atleast in part, one or more parent metal boron-containing compounds,which compounds include a parent metal boride or a parent metal borocarbide, or both, and typically also may include a parent metal carbide.It is also disclosed that the mass comprising boron carbide which is tobe infiltrated may also contain one or more inert fillers mixed with theboron carbide. Accordingly, by combining an inert filler, the resultwill be a composite body having a matrix produced by the reactiveinfiltration of the parent metal, said matrix comprising at least oneboron-containing compound, and the matrix may also include a parentmetal carbide, the matrix embedding the inert filler. It is furthernoted that the final composite body product in either of theabove-discussed embodiments (i.e., filler or no filler) may include aresidual metal as at least one metallic constituent of the originalparent metal.

Broadly, in the disclosed method of Patent '130, a mass comprising boroncarbide and, optionally, one or more of a boron donor material and acarbon donor material, is placed adjacent to or in contact with a bodyof molten metal or metal alloy, which is melted in a substantially inertenvironment within a particular temperature envelope. The molten metalinfiltrates the mass comprising boron carbide and reacts with at leastthe boron carbide to form at least one reaction product. The boroncarbide (and/or the boron donor material and/or the carbon donormaterial) is reducible, at least in part, by the molten parent metal,thereby forming the parent metal boron-containing compound (e.g., aparent metal boride and/or boro compound under the temperatureconditions of the process). Typically, a parent metal carbide is alsoproduced, and in certain cases, a parent metal boro carbide is produced.At least a portion of the reaction product is maintained in contact withthe metal, and molten metal is drawn or transported toward the unreactedmass comprising boron carbide by a wicking or a capillary action. Thistransported metal forms additional parent metal boride, carbide, and/orboro carbide and the formation or development of a ceramic body iscontinued until either the parent metal or mass comprising boron carbidehas been consumed, or until the reaction temperature is altered to beoutside of the reaction temperature envelope. The resulting structurecomprises one or more of a parent metal boride, a parent metal borocompound, a parent metal carbide, a metal (which, as discussed in Patent'130, is intended to include alloys and intermetallics), or voids, orany combination thereof. Moreover, these several phases may or may notbe interconnected in one or more dimensions throughout the body. Thefinal volume fractions of the boron-containing compounds (i.e., borideand boron compounds), carbon-containing compounds, and metallic phases,and the degree of interconnectivity, can be controlled by changing oneor more conditions, such as the initial density of the mass comprisingboron carbide, the relative amounts of boron carbide and parent metal,alloys of the parent metal, dilution of the boron carbide with a filler,the amount of boron donor material and/or carbon donor material mixedwith the mass comprising boron carbide, temperature, and time.Preferably, conversion of the boron carbide to the parent metal boride,parent metal boro compound(s) and parent metal carbide is at least about50%, and most preferably at least about 90%.

The typical environment or atmosphere which was utilized in Patent '130,was one which is relatively inert or unreactive under the processconditions. Particularly, it was disclosed that an argon gas, or avacuum, for example, would be suitable process atmospheres. Stillfurther, it was disclosed that when zirconium was used as the parentmetal, the resulting composite comprised zirconium diboride, zirconiumcarbide, and residual zirconium metal. It was also disclosed that whenaluminum parent metal was used with the process, the result was analuminum boro carbide such as Al₃ B₄₈ C₂, AlB₁₂ C₂ and/or AlB₂₄ C₄, withaluminum parent metal and other unreacted unoxidized constituents of theparent metal remaining. Other parent metals which were disclosed asbeing suitable for use with the processing conditions included silicon,titanium, hafnium, lanthanum, iron, calcium, vanadium, niobium,magnesium, and beryllium.

Still further, it is disclosed that by adding a carbon donor material(e.g., graphite powder or carbon black) and/or a boron donor material(e.g., a boron powder, silicon borides, nickel borides and iron borides)to the mass comprising boron carbide, the ratio of parentmetal-boride/parent metal-carbide can be adjusted. For example, ifzirconium is used as the parent metal, the ratio of ZrB₂ /ZrC can bereduced if a carbon donor material is utilized (i.e., more ZrC isproduced due to the addition of a carbon donor material in the mass ofboron carbide) while if a boron donor material is utilized, the ratio ofZrB₂ /ZrC can be increased (i.e., more ZrB₂ is produced due to theaddition of a boron donor material in the mass of boron carbide). Stillfurther, the relative size of ZrB₂ platelets which are formed in thebody may be larger than platelets that are formed by a similar processwithout the use of a boron donor material. Thus, the addition of acarbon donor material and/or a boron donor material may also affect themorphology of the resultant material.

In another related Patent, specifically, U.S. Pat. No. 4,915,736(hereinafter referred to as "Patent '736"), issued in the names of TerryDennis Claar and Gerhard Hans Schiroky, on Apr. 10, 1990, and entitled"A Method of Modifying Ceramic Composite Bodies By a CarburizationProcess and Articles Made Thereby", additional modification techniquesare disclosed. Specifically, Patent '736 discloses that a ceramiccomposite body made in accordance with the teachings of, for example,Patent '130, can be modified by exposing the composite to a gaseouscarburizing species. Such a gaseous carburizing species can be producedby, for example, embedding the composite body in a graphitic bedding andreacting at least a portion of the graphitic bedding with moisture oroxygen in a controlled atmosphere furnace. However, the furnaceatmosphere should comprise typically, primarily, a non-reactive gas suchas argon. It is not clear whether impurities present in the argon gassupply the necessary O₂ for forming a carburizing species, or whetherthe argon gas merely serves as a vehicle which contains impuritiesgenerated by some type of volatilization of components in the graphiticbedding or in the composite body. In addition, a gaseous carburizingspecies could be introduced directly into a controlled atmospherefurnace during heating of the composite body.

Once the gaseous carburizing species has been introduced into thecontrolled atmosphere furnace, the setup should be designed in such amanner to permit the carburizing species to be able to contact at leasta portion of the surface of the composite body buried in the looselypacked graphitic powder. It is believed that carbon in the carburizingspecies, or carbon from the graphitic bedding, will dissolve into theinterconnected zirconium carbide phase, which can then transport thedissolved carbon throughout substantially all of the composite body, ifdesired, by a vacancy diffusion process. Moreover, Patent '736 disclosesthat by controlling the time, the exposure of the composite body to thecarburizing species and/or the temperature at which the carburizationprocess occurs, a carburized zone or layer can be formed on the surfaceof the composite body. Such process could result in a hard,wear-resistant surface surrounding a core of composite material having ahigher metal content and higher fracture toughness.

Thus, if a composite body was formed having a residual parent metalphase in the amount of between about 5-30 volume percent, such compositebody could be modified by a post-carburization treatment to result infrom about 0 to about 2 volume percent, typically about 1/2 to about 2volume percent, of parent metal remaining in the composite body.

U.S. Pat. No. 4,885,131 (hereinafter "Patent '131"), issued in the nameof Marc S. Newkirk on Dec. 5, 1989, and entitled "Process For PreparingSelf-Supporting Bodies and Products Produced Thereby", disclosesadditional reactive infiltration formation techniques. Specifically,Patent '131 discloses that self-supporting bodies can be produced by areactive infiltration of a parent metal into a mixture of a bed or masscomprising a boron donor material and a carbon donor material. Therelative amounts of reactants and process conditions may be altered orcontrolled to yield a body containing varying volume percents ofceramic, metals, ratios of one ceramic or another and porosity.

In another related patent application, specifically, copending U.S.patent application Ser. No. 07/296,770 (which is equivalent to EPOPublication No. 0 383 715, published Aug. 22, 1991) (hereinafterreferred to as "Application '770"), filed in the names of Terry DennisClaar et al., on Jan. 13, 1989, and entitled "A Method of ProducingCeramic Composite Bodies", additional reactive infiltration formationtechniques are disclosed. Specifically, Application '770 disclosesvarious techniques for shaping a bed or mass comprising boron carbideinto a predetermined shape and thereafter reactively infiltrating thebed or mass comprising boron carbide to form a self-supporting body of adesired size and shape.

U.S. Pat. No. 5,011,063 (hereinafter referred to as "Patent '063"),which issued on Apr. 30, 1991, from U.S. patent application Ser. No.07/560,491, filed Jul. 23, 1990, which is a continuation of U.S. patentapplication Ser. No. 07/296,837 (which is equivalent to EPO PublicationNo. 0 378 501, which published Jul. 18, 1990), filed in the name ofTerry Dennis Claar on Jan. 13, 1989, and entitled "A Method of Bonding ACeramic Composite Body to a Second Body and Articles Produced Thereby",discloses various bonding techniques for bonding self-supporting bodiesto second materials. Particularly, this patent discloses that a bed ormass comprising one or more boron-containing compounds is reactivelyinfiltrated by a molten parent metal to produce a self-supporting body.Moreover, residual or excess metal is permitted to remain bonded to theformed self-supporting body. The excess metal is utilized to form a bondbetween the formed self-supporting body and another body (e.g., a metalbody or a ceramic body of any particular size or shape).

The reactive infiltration of a parent metal into a bed or masscomprising boron nitride is disclosed in U.S. Pat. No. 4,904,446(hereinafter "Patent '446"), issued in the names of Danny Ray White etal., on Feb. 27, 1990, and entitled "Process for PreparingSelf-Supporting Bodies and Products Made Thereby". Specifically, thispatent discloses that a bed or mass comprising boron nitride can bereactively infiltrated by a parent metal. A relative amount of reactantsand process conditions may be altered or controlled to yield a bodycontaining varying volume percents of ceramic, metal and/or porosity.Additionally, the self-supporting body which results comprises aboron-containing compound, a nitrogen-containing compound and,optionally, a metal. Additionally, inert fillers may be included in theformed self-supporting body.

A further post-treatment process for modifying the properties ofproduced ceramic composite bodies is disclosed in U.S. Pat. No.5,004,714 (hereinafter "Patent '714"), which issued on Apr. 2, 1991,from U.S. patent application Ser. No. 07/296,966 (which is equivalent toEPO Publication No. 0 378 503, which published Jul. 18, 1990), filed inthe names of Terry Dennis Claar et al., on Jan. 13, 1989, and entitled"A Method of Modifying Ceramic Composite Bodies By Post-TreatmentProcess and Articles Produced Thereby". Specifically, Patent '714discloses that self-supporting bodies produced by a reactiveinfiltration technique can be post-treated by exposing the formed bodiesto one or more metals and heating the exposed bodies to modify at leastone property of the previously formed composite body. One specificexample of a post-treatment modification step includes exposing a formedbody to a siliconizing environment.

U.S. Pat. No. 5,019,539 (hereinafter "Patent '539"), which issued on May28, 1991, from U.S. patent application Ser. No. 07/296,961 (which isequivalent to EPO Publication No. 0 378 504, which published Jul. 18,1990), filed in the names of Terry Dennis Claar et al., on Jan. 13,1989, and entitled "A Process for Preparing Self-Supporting BodiesHaving Controlled Porosity and Graded Properties and Products ProducedThereby", discloses reacting a mixture of powdered parent metal with abed or mass comprising boron carbide and, optionally, one or more inertfillers. Additionally, it is disclosed that both a powdered parent metaland a body or pool of molten parent metal can be induced to react with abed or mass comprising boron carbide. The body which is produced is abody which has controlled or graded properties.

The disclosures of each of the above-discussed Commonly Owned U.S.Patent Applications and Patents are herein expressly incorporated byreference.

SUMMARY OF THE INVENTION

In accordance with a first step of the present invention, somewhatporous self-supporting ceramic bodies are produced by utilizing a parentmetal infiltration and reaction process (i.e. reactive infiltration) inthe presence of a bed or mass comprising a boron donor material and acarbon donor material and/or a boron donor material and a nitrogen donormaterial, such as, for example, boron carbide or boron nitride. Such bedor mass is infiltrated by molten parent metal, and the bed may becomprised entirely of boron carbide, boron nitride, and/or mixtures ofboron donor materials and carbon donor materials and/or boron donormaterials and nitrogen donor materials. Depending on the particularreactants involved in the reactive infiltration, the resulting bodieswhich are produced comprise one or more reaction products such as one ormore parent metal boron-containing compounds, and/or one or more parentmetal carbon-containing compounds and/or one or more parent metalnitrogen-containing compounds, etc. Alternatively, the mass to beinfiltrated may contain one or more inert fillers admixed therewith toproduce a composite by reactive infiltration, which composite comprisesa matrix of one or more of the aforementioned reaction products and alsomay include residual unreacted or unoxidized constituents of the parentmetal. The filler material may be embedded by the formed matrix. Thefinal product may include a metallic component which may comprise as oneor more metallic constituents of the parent metal. Still further, insome cases it may be desirable to add a carbon donor material (i.e., acarbon-containing compound) and/or a boron donor material (i.e., aboron-containing compound), and/or a nitrogen donor material (i.e., anitrogen-containing compound) to the bed or mass which is to beinfiltrated to modify, for example, the relative amounts of one formedreaction product to another, thereby modifying resultant mechanicalproperties of the composite body. Still further, the reactantconcentrations and process conditions may be altered or controlled toyield a body containing varying volume percents of ceramic compounds,metal and/or porosity.

The self-supporting body which is produced in accordance with the firststep of the invention needs to be relatively porous so that the formedporosity can be later filled by metal. It is desirable for at least someof the porosity to be at least partially interconnected, and in onepreferred embodiment, it is desirable for substantially all of theporosity to be interconnected. The porosity can be formed by a number ofdifferent techniques, including: not permitting complete reaction of thereactants to occur; providing insufficient amounts of at least onereactant; incorporating at least one material which will cause porosityto be formed; utilizing a combination of parent metal and mass to bereactively infiltrated which does not substantially completely react;incorporating a powdered parent metal in the mass to be reactivelyinfiltrated; etc.

Broadly, in accordance with the first step of the method according tothis invention, the bed or mass which is to be reactively infiltratedmay be placed adjacent to or in contact with a body of molten parentmetal or parent metal alloy, which is melted in a substantially inertenvironment within a particular temperature envelope. Appropriate parentmetals for use in the present invention include such metals aszirconium, titanium, hafnium, aluminum, vanadium, chromium, niobium,etc., and particularly preferred parent metals include zirconium,titanium and hafnium. The molten metal infiltrates the mass and reactswith at least one constituent of the bed or mass to be infiltrated toform one or more reaction products. At least a portion of the formedreaction product is maintained in contact with the metal, and moltenmetal is drawn or transported toward the remaining unreacted mass by awicking or capillary action. This transported metal forms additionalreaction product upon contact with the remaining unreacted mass, and theformation or development of a ceramic body is continued until the parentmetal or remaining unreacted mass has been consumed, or until thereaction temperature is altered to be outside the reaction temperatureenvelope. The resulting structure comprises, depending upon theparticular materials comprising the bed or mass which is to bereactively infiltrated, one or more of a parent metal boride, a parentmetal boro compound, a parent metal carbide, a parent metal nitride, ametal (which as used herein is intended to include alloys andintermetallics), or voids, or a combination thereof, and these severalphases may or may not be interconnected in one or more dimensions. Thefinal volume fractions of the reaction products and metallic phases, andthe degree of interconnectivity, can be controlled by changing one ormore conditions, such as the initial density of the mass to bereactively infiltrated, the relative amounts and chemical composition ofthe materials contained within the mass which is to be reactivelyinfiltrated, the amount of parent metal provided for reaction, thecomposition of the parent metal, the presence and amount of one or morefiller materials, temperature, time, etc.

Typically, the mass to be reactively infiltrated should be at leastsomewhat porous so as to allow for wicking the parent metal through thereaction product. Wicking occurs apparently either because any volumechange on reaction does not fully close off pores through which parentmetal can continue to wick, or because the reaction product remainspermeable to the molten metal due to such factors as surface energyconsiderations which render at least some of its grain boundariespermeable to the parent metal.

In another aspect of the first step of the invention, a composite isproduced by the transport of molten parent metal into the bed or masswhich is to be reactively infiltrated which has admixed therewith one ormore inert filler materials. In this embodiment, one or more suitablefiller materials are mixed with the bed or mass to be reactivelyinfiltrated. The resulting self-supporting ceramic-metal composite thatis produced typically comprises a dense microstructure which comprises afiller embedded by a matrix comprising at least one parent metalreaction product, and also may include a substantial quantity of metal.Typically, only a small amount of material (e.g., a small amount ofboron carbide, etc.) is required to promote the reactive infiltrationprocess. Thus, the resulting matrix can vary in content from onecomposed primarily of metallic constituents thereby exhibiting certainproperties characteristic of the parent metal, to cases where a highconcentration of reaction product is formed, which dominates theproperties of the matrix. The filler may serve to enhance the propertiesof the composite, lower the raw materials cost of the composite, ormoderate the kinetics of the reaction product formation reactions andthe associated rate of heat evolution. The precise starting amounts andcomposition of materials utilized in the reactive infiltration processcan be selected so as to result in a desirable body which is compatiblewith the second step of the invention.

In another aspect of the first step of the present invention, thematerial to be reactively infiltrated is shaped into a preformcorresponding to the geometry of the desired final composite. Reactiveinfiltration of the preform by the molten parent metal results in acomposite having the net shape or near net shape of the preform, therebyminimizing expensive final machining and finishing operations. Moreover,to assist in reducing the amount of final machining and finishingoperations, a barrier material can at least partially, or substantiallycompletely, surround the preform. For example, a graphite material(e.g., a graphite mold, a graphite tape product, a graphite coating,etc.) is particularly useful as a barrier for such parent metals aszirconium, titanium, or hafnium, when used in combination with preformsmade of, for example, boron carbide, boron nitride, boron and carbon.Still further, by placing an appropriate number of through-holes havinga particular size and shape in the aforementioned graphite mold, theamount of porosity which typically occurs within a composite bodymanufactured according to the first step of the present invention, canbe reduced. Typically, a plurality of holes is placed in a bottomportion of the mold, or that portion of the mold toward which reactiveinfiltration occurs. The holes function as a venting means which permitthe removal of, for example, argon gas which has been trapped in thepreform as the parent metal reactive infiltration front infiltrates thepreform.

Still further, the procedures discussed above herein in the Section"Discussion of Commonly Owned U.S. Patents and Patent Applications" maybe applicable in connection with the first step of the presentinvention.

Once a self-supporting body has been formed in accordance with the firststep of the present invention, then the second step of the presentinvention is put into effect. Specifically, the second step of thepresent invention involves infiltrating the porosity which was formed ina self-supporting body made in accordance with the first step of theinvention with a metal having a composition which is substantiallysimilar to, or substantially different from, the parent metal utilizedto form reaction product in the first step of the invention.

A primary selection criteria for selecting the metal which is to fill atleast a portion of the porosity formed in the body in accordance withthe first step of the present invention, is that the metal should becapable of desirably infiltrating the porosity without the requirementfor the application of external pressure. However, in some cases, it maybe desirable to assist (e.g., by application of pressure) a certainmetal(s) to infiltrate the formed porosity due to particular desirablecharacteristics which may be achieved by using that specific metal.Moreover, compatibility of the metal with the remaining constituents inthe formed body is also important. Thus, selection of the metal shouldbe governed by the ability to insert a particular metal into theporosity of a formed body, as well as the desired properties of theultimately formed body. Typically, these factors need to be balanced byan artisan of ordinary skill to achieve the precise desirablecomposition of metal, as well as an acceptable technique for placing themetal in the porosity of a first formed self-supporting body. Suitablemetals which may be utilized to infiltrate the porosity of a porousself-supporting body may include, for example, zirconium, titanium,hafnium, aluminum, vanadium, niobium, tantalum, chromium, manganese,iron, cobalt, nickel, copper, zinc, etc., and alloys and combinationsthereof.

In a preferred embodiment of the invention, the metal, when made molten,is placed into the porosity of the first formed body. Specifically, bycontacting a molten metal with the porosity in a formed self-supportingbody, the molten metal can be caused to wick into such porosity by awetting phenomenon. Such wicking action is desirable because theapplication of external pressure is not required to assist in theinfiltration process.

DEFINITIONS

As used in this specification and the appended claims, the terms beloware defined as follows:

"Parent metal" refers to that metal, e.g. zirconium, titanium, hafnium,etc., which is the precursor to the polycrystalline oxidation reactionproduct, that is, the parent metal boride, parent metal carbide, parentmetal nitride, or other parent metal compound, and includes that metalas a pure or relatively pure metal, a commercially available metalhaving impurities and/or alloying constituents therein, and an alloy inwhich that metal precursor is the major constituent; and when a specificmetal is mentioned as the parent metal, e.g. zirconium, titanium,hafnium, etc., the metal identified should be read with this definitionin mind unless indicated otherwise by the context.

"Parent metal boride" and "parent metal boro compounds" mean a reactionproduct containing boron formed upon reaction between a boron donormaterial, such as boron carbide or boron nitride, and the parent metaland includes a binary compound of boron with the parent metal as well asternary or higher order compounds.

"Parent metal nitride" means a reaction product containing nitrogenformed upon reaction of a nitrogen donor material, such as boronnitride, and the parent metal.

"Parent metal carbide" means a reaction product containing carbon formedupon reaction of a carbon donor material, such as boron carbide, and theparent metal.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic view of the lay-up used to fabricate a plateletreinforced composite body; and

FIG. 2 is a schematic view of the lay-up used to reinfiltrate the formedplatelet reinforced composite body with additional metal.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

In accordance with a first step of the present invention, somewhatporous self-supporting ceramic bodies are produced by utilizing a parentmetal infiltration and reaction process (i.e. reactive infiltration) inthe presence of a bed or mass comprising a boron donor and a carbondonor and/or a boron donor and a nitrogen donor, such as, for example,boron carbide or boron nitride. Such bed or mass is infiltrated bymolten parent metal, and the bed may be comprised entirely of boroncarbide, boron nitride, and/or mixtures of boron donor materials andcarbon donor materials and/or boron donor materials and nitrogen donormaterials. Depending on the particular reactants involved in thereactive infiltration, the resulting bodies which are produced compriseone or more reaction products such as one or more parent metalboron-containing compounds, and/or one or more parent metalcarbon-containing compounds and/or one or more parent metalnitrogen-containing compounds, etc. Alternatively, the mass to beinfiltrated may contain one or more inert fillers admixed therewith toproduce a composite by reactive infiltration, which composite comprisesa matrix of one or more of the aforementioned reaction products and alsomay include residual unreacted or unoxidized constituents of the parentmetal. The filler material may be embedded by the formed matrix. Thefinal product may include a metallic component which may comprise one ormore metallic constituents of the parent metal. Still further, in somecases it may be desirable to add a carbon donor material (i.e., acarbon-containing compound) and/or a boron donor material (i.e., aboron-containing compound) and/or a nitrogen donor material (i.e., anitrogen-containing compound) to the bed or mass which is to beinfiltrated to modify, for example, the relative amounts of one formedreaction product to another, thereby modifying resultant mechanicalproperties of the composite body. Still further, the reactantconcentrations and process conditions may be altered or controlled toyield a body containing varying volume percents of ceramic compounds,metal and/or porosity.

The self-supporting body which is produced in accordance with the firststep of the invention needs to be relatively porous so that the formedporosity can be later filled by metal. It is desirable for at least someof the porosity to be at least partially interconnected, and in onepreferred embodiment, it is desirable for substantially all of theporosity to be interconnected. The porosity can be formed by a number ofdifferent techniques, including: not permitting complete reaction of thereactants to occur; providing insufficient amounts of at least onereactant; incorporating at least one material which will cause porosityto be formed; utilizing a combination of parent metal and mass to bereactively infiltrated which does not substantially completely react;incorporating a powdered parent metal in the mass to be reactivelyinfiltrated; etc.

Broadly, in accordance with the first step of the method according tothis invention, the bed or mass which is to be reactively infiltratedmay be placed adjacent to or contacted with a body of molten metal ormetal alloy, which is melted in a substantially inert environment withina particular temperature envelope. Appropriate parent metals for use inthe present invention include such metals as zirconium, titanium,hafnium, aluminum, vanadium, chromium, niobium, etc., and particularlypreferred metals include zirconium, titanium and hafnium. The moltenmetal infiltrates the mass and reacts with at least one constituent ofthe bed or mass to be infiltrated to form one or more reaction products.At least a portion of the formed reaction product is maintained incontact with the metal, and molten metal is drawn or transported towardthe remaining unreacted mass by a wicking or capillary action. Thistransported metal forms additional reaction product upon contact withthe remaining unreacted mass, and the formation or development of aceramic body is continued until the parent metal or remaining unreactedmass has been consumed, or until the reaction temperature is altered tobe outside the reaction temperature envelope. The resulting structurecomprises, depending upon the particular materials comprising the bed ormass which is to be reactively infiltrated, one or more of a parentmetal boride, a parent metal boro compound, a parent metal carbide, aparent metal nitride, a metal (which as used herein is intended toinclude alloys and intermetallics), or voids, or a combination thereof,and these several phases may or may not be interconnected in one or moredimensions. The final volume fractions of the reaction products andmetallic phases, and the degree of interconnectivity, can be controlledby changing one or more conditions, such as the initial density of themass to be reactively infiltrated, the relative amounts and chemicalcomposition of the materials contained within the mass which is to bereactively infiltrated, the amount of parent metal provided forreaction, the composition of the parent metal, the presence and amountof one or more filler materials, temperature, time, etc.

Typically, the mass to be reactively infiltrated should be at leastsomewhat porous so as to allow for wicking the parent metal through thereaction product. Wicking occurs apparently either because any volumechange on reaction does not fully close off pores through which parentmetal can continue to wick, or because the reaction product remainspermeable to the molten metal due to such factors as surface energyconsiderations which render at least some of its grain boundariespermeable to the parent metal.

In another aspect of the first step of the invention, a composite isproduced by the transport of molten parent metal into the bed or masswhich is to be reactively infiltrated which has admixed therewith one ormore inert filler materials. In this embodiment, one or more suitablefiller materials are mixed with the bed or mass to be reactivelyinfiltrated. The resulting self-supporting ceramic-metal composite thatis produced typically comprises a dense microstructure which comprises afiller embedded by a matrix comprising at least one parent metalreaction product, and also may include a substantial quantity of metal.Typically, only a small amount of material (e.g., a small amount ofboron carbide, etc.) is required to promote the reactive infiltrationprocess. Thus, the resulting matrix can vary in content from onecomposed primarily of metallic constituents thereby exhibiting certainproperties characteristic of the parent metal; to cases where a highconcentration of reaction product is formed, which dominates theproperties of the matrix. The filler may serve to enhance the propertiesof the composite, lower the raw materials cost of the composite, ormoderate the kinetics of the reaction product formation reactions andthe associated rate of heat evolution. The precise starting amounts andcomposition of materials utilized in the reactive infiltration processcan be selected so as to result in a desirable body which is compatiblewith the second step of the invention.

In another aspect of the first step of the present invention, thematerial to be reactively infiltrated is shaped into a preformcorresponding to the geometry of the desired final composite. Subsequentreactive infiltration of the preform by the molten parent metal resultsin a composite having the net shape or near net shape of the preform,thereby minimizing expensive final machining and finishing operations.Moreover, to assist in reducing the amount of final machining andfinishing operations, a barrier material can at least partially, orsubstantially completely, surround the preform. For example, a graphitematerial (e.g., a graphite mold, a graphite tape product, a graphitecoating, etc.) is particularly useful as a barrier for such parentmetals as zirconium, titanium, or hafnium, when used in combination withpreforms made of, for example, boron carbide, boron nitride, boron andcarbon. Still further, by placing an appropriate number of through-holeshaving a particular size and shape in the aforementioned graphite mold,the amount of porosity which typically occurs within a composite bodymanufactured according to the first step of the present invention, canbe reduced. Tpyically, a plurality of holes is placed in a bottomportion of the mold, or that portion of the mold toward which reactiveinfiltration occurs. The holes function as a venting means which permitthe removal of, for example, argon gas which has been trapped in thepreform as the parent metal reactive infiltration front infiltrates thepreform.

Still further, the procedures discussed above herein in the Section"Discussion of Commonly Owned U.S. Patents and Patent Applications" maybe applicable in connection with the first step of the presentinvention.

Once a self-supporting body has been formed in accordance with the firststep of the present invention, then the second step of the presentinvention is put into effect. Specifically, the second step of thepresent invention involves infiltrating the porosity which was formed ina self-supporting body made in accordance with the first step of theinvention with a metal having a composition which is substantiallysimilar to, or substantially different from, the parent metal utilizedto form reaction product in the first step of the invention.

A primary selection criteria for selecting the metal which is to fill atleast a portion of the porosity formed in the body in accordance withthe first step of the present invention, is that the metal should becapable of desirably infiltrating the porosity without the requirementfor the application of external pressure. However, in some cases, it maybe desirable to assist (e.g., by application of pressure) a certainmetal(s) to infiltrate the formed porosity due to particular desirablecharacteristics which may be achieved by using that specific metal.Moreover, compatibility of the metal with the remaining constituents inthe formed body is also important. Thus, selection of the metal shouldbe governed by the ability to insert a particular metal into theporosity of a formed body, as well as the desired properties of theultimately formed body. Typically, these factors need to be balanced byan artisan of ordinary skill to achieve the precise desirablecomposition of metal, as well as an acceptable technique for placing themetal in the porosity of a first formed self-supporting body. Suitablemetals which may be utilized to infiltrate the porosity of a porousself-supporting body may include, for example, zirconium, titanium,hafnium, aluminum, vanadium, niobium, tantalum, chromium, manganese,iron, cobalt, nickel, copper, zinc, etc., and alloys and combinationsthereof.

In a preferred embodiment of the invention, the metal, when made molten,is placed into the porosity of the first formed body. Specifically, bycontacting a molten metal with the porosity in a formed self-supportingbody, the molten metal can be caused to wick into such porosity by awetting phenomenon. Such wicking action is desirable because theapplication of external pressure is not required to assist in theinfiltration process.

The following are examples of the present invention. The Examples areintended to be illustrative of various aspects of the present invention,however, these examples should not be construed as limiting the scope ofthe invention.

EXAMPLE 1

This Example demonstrates a method for reducing the amount of porosityin a porous platelet reinforced composite body by reinfiltrating saidbody with additional parent metal. The lay-up used to produce theplatelet reinforced composite body is shown in FIG. 1, while the lay-upused to reinfiltrate said body with additional parent metal is shown inFIG. 2.

About 53.43 grams of 100 grit TETRABOR® boron carbide particulate(Engineered Ceramics, New Canaan, Conn.), having an average particlesize of about 173 microns, about 3.39 grams of ACRAWAX® C wax binder(Lonza, Inc., Fair Lawn, N.J.), and about 172.41 grams of titaniumparticulate (-100 mesh, Consolidated Astronautics, Saddle River, N.J.),having substantially all particles smaller than about 150 microns, wereplaced into an approximately one liter NALGENE® plastic jar (NalgeCompany, Rochester, N.Y.) and roll mixed on a mill rack for about 1 1/2hours to form an admixture.

A dry pressing die made of tool steel was set up on a vibration tableand vibration was commenced. The roll mixed powder admixture was slowlypoured into the die. The degree of vibration was adjusted such thatparticle movement could just barely be detected under the action of thevibration. After substantially all of the roll mixed powder admixturehad been introduced into the die cavity and vibrated, the top pressingpunch was inserted and the pressing die was placed into a hand operatedhydraulic press. The roll mixed powder admixture was pressed under anapplied pressure of about 10,000 psi (69 MHPa) to make a preform. Thedry pressed powder preform 10 which was removed from the die measuredabout 2.0 inches (51 mm) square by about 1.0 inch (25 mm) thick.

As shown in FIG. 1, the dry pressed preform 10 was then placed into aGrade ATJ graphite crucible 12 (Union Carbide Company, Carbon ProductsDivision, Cleveland, Ohio), having interior dimensions measuring about2.0 inches (51 mm) square by about 3.0 inches (76 mm) high. The graphitecrucible 12 containing the dry pressed preform 10 was then placed into avacuum furnace. The furnace chamber was evacuated to about 30 inches(762 mm) of mercury vacuum and backfilled with argon gas. Afterrepeating this procedure, an argon gas flow rate of about two liters perminute was established through the furnace chamber at an overpressure ofabout 1 psi (7 kPa). The furnace temperature was then increased fromabout room temperature to a temperature about 200° C. at a rate of about50° C. per hour. Upon reaching a temperature of about 200° C., thetemperature was then increased to about 570° C. at a rate of about 30°C. per hour. After maintaining a temperature of about 570° C. for aboutone hour, substantially all of the wax binder had volatilized.Accordingly, the temperature was then decreased to about roomtemperature at a rate of about 100° C. per hour. After cooling down toabout room temperature, the graphite crucible 12 and its contents wereremoved from the furnace.

The graphite crucible 12 and the dry pressed preform 10 was then placedinto a graphite boat 14 measuring about 6.0 inches (150 mm) square andabout 4.0 inches (102 mm) high. About 200 grams of nuclear gradezirconium sponge 16 (Western Zirconium, Ogden, Utah) was placed in thegraphite boat 14 around the graphite crucibles 12 to serve as agettering agent for oxygen and nitrogen impurities. The graphite boat 14and its contents were then placed into a vacuum furnace. The furnacechamber was evacuated using a mechanical roughing pump to about 30inches (762 mm) of mercury vacuum and backfilled with argon gas. Thechamber was evacuated a second time with the roughing pump, but thistime followed by a high vacuum source to produce a working operatingpressure of less than 10⁻⁴ torr. The furnace temperature was thenincreased from about room temperature to a temperature of about 250° C.at a rate of about 100° C. per hour. Upon reaching a temperature ofabout 250° C, the temperature was then increased to about 1800° C. at arate of about 400° C. per hour. Upon reaching an intermediatetemperature of about 1000° C., the high vacuum source was disconnectedfrom the furnace chamber and heating continued throughout the rest ofthe run utilizing the mechanical roughing pump as the vacuum source.After maintaining a temperature of about 1800° C. for about two hours,the furnace chamber was cooled to about room temperature at a rate ofabout 350° C. per hour. The graphite boat 14 and its contents wereremoved from the furnace. The formed platelet reinforced composite,weighing about 164 grams, and measuring about 2.0 inches (51 mm) squareby about 1.0 inch (25 mm) thick, was removed from the graphite crucible12. The calculated bulk density of the composite was about 2.52 gramsper cubic centimeter, corresponding to a theoretical density of about66.5%.

As shown in FIG. 2, the platelet reinforced composite body 18 withapproximately 33.5% porosity was then placed back into the graphitecrucible 12. About 142 grams of titanium particulate 20 (-80 +325 mesh,Consolidated Astronautics, Saddle River, N.J.), having substantially allparticles between about 45 microns and about 177 microns in diameter,was placed into the graphite crucible 12 on top of the porous body ofplatelet reinforced composite material 18 and levelled to form a lay-up.The lay-up was placed into a graphite boat 14 measuring about 6 inches(52 mm), by about 6 inches (52 mm), by about 4 inches (102 mm) high.About 200 grams of nuclear grade zirconium sponge 16 (Western Zirconium,Ogden, Utah) was placed in the graphite boat 14 around the graphitecrucible 12 to getter any gaseous impurities.

The graphite boat 14 and its contents were placed into a vacuum furnace.The furnace chamber was evacuated to about 30 inches (762 mm) of mercuryvacuum and then backfilled with argon gas. A second evacuation wasperformed to about 30 inches (762 mm) of mercury vacuum, at which pointa high vacuum source was connected to the furnace chamber to pump thechamber down to a final working pressure of about 4×10⁻⁴ torr. Thefurnace temperature was then increased from about room temperature to afinal temperature of about 1900° C. at a rate of about 400° C. per hour.At an intermediate temperature of about 1000° C., however, the highvacuum source was isolated from the furnace chamber and for theremainder of the run the chamber was pumped with the mechanical roughingpump. After maintaining a temperature of about 1900° C. for about twohours, the furnace temperature was then decreased to substantially roomtemperature at a rate of about 350° C. per hour. Upon cooling to aboutroom temperature, the graphite boat and its contents were removed fromthe furnace chamber. The body which was recovered from the graphitecrucible was found to weigh about 302 grams and measured about 2.04inches (52 mm) square by about 1.14 inches (29 mm) high. The bulkdensity was calculated to be about 3.86 grams per cubic centimeter,which corresponds to a theoretical density of about 86.0 %. Thereinfiltration of the porous platelet reinforced composite body 18 withadditional molten parent metal 20 increased the density of the body andconcomitantly reduced the volume fraction of porosity from about 33.5%to about 14.2%.

I claim:
 1. A method for producing a self-supporting body,comprising:providing at least one porous self-supporting body which ismade by a process comprising: (i) forming an admixture comprising apowdered parent metal and a mass comprising at least one boron donormaterial, wherein an amount of said mass comprising at least one borondonor material of said admixture is insufficient to react completelywith all of said parent metal; (ii) heating said parent metal in asubstantially inert atmosphere to a temperature above its melting pointto form molten parent metal contacting said mass comprising at least oneboron material; (iii) maintaining said temperature for a time sufficientto permit reaction of said molten parent metal with said mass comprisingat least one boron donor material to form at least one boron-containingcompound; (iv) continuing said reaction for a time sufficient to producesaid at least one porous self-supporting body; contacting at least aportion of said at least one porous self-supporting body with at leastone body of molten metal having a composition which is substantiallysimilar to said parent metal; infiltrating at least a portion of theporosity of said at least one porous self-supporting body with moltenmetal; and continuing said infiltration for a time sufficient toinfiltrate a desired amount of porosity, thereby forming aself-supporting body which is more dense than said at least one porousself-supporting body.
 2. The method of claim 1, wherein said parentmetal comprises at least one metal selected from the group consisting oftitanium, zirconium, hafnium, aluminum, vanadium, chromium and niobium,and alloys thereof.
 3. The method of claim 2, wherein said parent metalcomprises at least one metal selected from the group consisting ofzirconium, titanium and hafnium.
 4. The method of claim 1, wherein saidat least one porous self-supporting body comprises at least twomaterials selected from the group consisting of at least oneboron-containing material, at least one carbon-containing material, atleast one nitrogen-containing material and metal.
 5. The method of claim1, wherein said mass further comprises at least one material selectedfrom the group consisting of a carbon donor material and a nitrogendonor material.
 6. The method of claim 5, wherein said mass furthercomprises an inert filler material.
 7. The method of claim 5, whereinsaid mass comprises at least one material selected from the groupconsisting of boron, carbon, boron carbide and boron nitride.
 8. Themethod of claim 1, further comprising incorporating a porosity formingmaterial in said mass.
 9. The method of claim 1, wherein at least aportion of said mass remains unreacted in said at least one producedporous self-supporting body.
 10. The method of claim 1, wherein saidporous body comprises at least two materials selected from the groupconsisting of a parent metal boride, a parent metal boro compound, aparent metal carbide, a parent metal nitride, residual metal and voids.11. The method of claim 1, wherein said admixture comprises a preform.12. The method of claim 1, further comprising providing a barrier incontact with at least one surface of said admixture.
 13. The method ofclaim 12, wherein said barrier comprises a graphite material in the formof a graphite mold, a graphite tape or a graphite coating.
 14. Themethod of claim 1, wherein said molten metal infiltrates said at leastone porous self-supporting body without the requirement for theapplication of pressure.
 15. The method of claim 1, wherein said moltenmetal comprises at least one metal selected from the group consisting ofzirconium, titanium, hafnium, aluminum, vanadium, niobium, tantalum,chromium, manganese, iron, cobalt, nickel, copper, zinc, and alloys andcombinations thereof.
 16. The method of claim 1, wherein said parentmetal comprises a metal selected from the group consisting of zirconium,titanium and hafnium, and said mass comprises a material selected fromthe group consisting of boron carbide and boron nitride.
 17. The methodof claim 1, wherein said parent metal comprises zirconium, said fillercomprises boron carbide and said molten metal comprises titanium. 18.The method of claim 1, further comprising reactively infiltrating saidadmixture from an external body of parent metal contacting saidadmixture.